Because tailgate assemblies can be damaged by the difficult tasks performed by off-road, heavy-duty trucks, the bodies of the trucks have traditionally been designed to function without tailgates. But, as a result of a desire to increase capacity, tailgate assemblies have been designed which are capable of withstanding the harsh working environment of off-road trucks. For example, in U.S. Pat. No. 3,751,112 to Hagenbuch a tailgate assembly includes two side plates that are pivotally connected to the sides of the body. In order to raise the tailgate upon the pivoting of the body, a pair of outriggers mounted to the frame of the truck, secure one end of a pair of chains (or cables) which cause the tailgate to rotate when the body is dumped.
By providing outriggers, the chains or cables which rotate the tailgate are attached at a point on the frame which extends out beyond the side of the body. By attaching one end of the chains at the end of the outriggers, the chains are not biased around the bottom edge of the body, which would cause damage to both the chains and the truck body.
But, because the outriggers are a cantilever and extend from the frame to a point beyond the sides of the body, the outriggers may be plagued with problems under certain conditions. For instance, because of limited available space on the frames of some truck models, the area of the base of the outriggers (where they attach to the frame) may be smaller than desired. The strength of the outriggers may be compromised if the bases of the outriggers have areas too small for the outriggers to withstand the forces exerted at the ends of the outriggers as the tailgate is rotated when the body pivots. As a result of the area of the base being too small, the outriggers may be susceptible to bending after repeated use. Finally, some manufacturers of trucks are reluctant to honor warranties if the frame of the truck has been subjected to welding as is required to mount the outriggers for the addition of a tailgate assembly.
One attempt at eliminating the potential problems of the use of outriggers to raise tailgates has been to use a shortened outrigger which does not extend beyond the sides of the body. By shortening the outrigger, the weakening caused by cantilevering is reduced. Because these outriggers do not extend beyond the sides of the body, the chains (or cables) are biased against the bottom edge of the body. When the body is pivoted to a dump position, the chains move relative to the bottom edge of the body. In order to prevent damage of the body or the chains as the body pivots in dumping, an elongated roller is mounted on each side of the body in order to protect the truck body and reduce wear of the chain.
Each roller is elongated because, as the body pivots, the chain changes position along the length of the sides of the body. As a result, in addition to the roller rotating as the chain pulls the tailgate to a raised position, the chain also walks along the length of the roller as the relative positions of the chain and roller change. Because of frictional forces, the chain does not walk smoothly along the length of the roller; instead, it moves in a jerking motion caused by the friction between the roller and chain holding the chain in a stationary lateral position until movement of the body and tailgate cause an angle in the chain which creates sufficient force to overcome the friction. As the body continues to pivot, the forces created by the bending chain increase until they again overcome the friction and the chain snaps to a new position. This jerking movement is extremely hard on both the rollers, the chains, the outrigger, the tailgate and the truck frame. As a result, the rollers and chains wear out quickly.
To overcome the foregoing wear problem, it is known to use a cable connection instead of a chain. Because a cable is composed of multiple strands of wire, frictional forces between strands become substantial if the elongated rollers bend the cable at too great an angle. In order to reduce the angle, the shortened outriggers are provided. Although the amount of extension of these shortened outriggers reduces problems inherent in a cantilevered support off the truck frame, the problems remain.
A related problem of tailgate assemblies that often causes damage is the undesirable "bouncing" or rotation of the assembly in response to a torque created about the tailgate s rotational axis by a moment arm between the rear axle of the truck and the tailgate's center of gravity created in response to oscillation of the rear axle relative to the surface of the road. This bouncing or banging of the tailgate assembly on the body of the truck may cause damage, depending on the length of the moment arm and the condition of the road. For truck bodies that extend substantially beyond the rear axle of the truck, the lever arm is sufficiently great that even relatively smooth roads will cause banging of the tailgate assembly that may be damaging.